This invention relates to a method for producing a tubular multi-layered porous barrier and more particularly, to a method for producing a tubular multi-layered porous barrier in which fine powder is deposited onto the inner and/or outer surfaces of a sintered tubular porous support member in a layer of uniform thickness to form a fine powder layer, and wherein the powder layer is compressed against said inner and/or outer surfaces of the support member to form a porous barrier layer on the support member.
When an isotope such as uranium is separated and enriched, the separation and enrichment have been effected by the so-called gas diffusion system utilizing a porous barrier. Such a porous barrier generally comprises a porous base formed by sintering metal or ceramic powder or by compression-molding organic synthetic resin powder such as fluoric resin and has a plurality of fine pores and especially, micro-fine pores of the average several 10 A - several hundred A. In order to effect the separation and enrichment of uranium efficiently, it is necessary to form as thin a porous barrier as possible. However, it is impossible to form a porous barrier which is excessively thin from the view point of strength. Thus, it is necessary to reinforce the thin porous barrier with a porous support member having a given thickness and a high gas permeability or with a wire net to thereby provide a multi-layered porous barrier structure.
On the other hand, when a multi-layered porous barrier is to be constructed in the form of a tube, such a tubular multi-layered porous barrier can be produced in any one of the prior art methods. Generally, the multi-layered tubular porous barrier is produced by bending a multi-layered porous barrier in the form of a sheet into a tube and then butt-welding the opposite side edges of the tube or lap-seaming the opposite side edges. However, when the porous barrier is formed of a ductile material such as metal or the like, the metal barrier can be shaped into a tubular construction, but when the porous barrier is formed of ceramic which has brittle, it is very difficult to shape such the barrier into a tube, because there is the possibility that a crack or cracks will develop in the barrier.
Even if the reinforcing support member is formed of a porous metal, the ductility of the reinforcing support member is substantially lower than that of a non-porous reinforcing support because of the presence of pores. Therefore, the porous reinforcing support member is subjected to limitations in the radius of curvature allowable for shaping the support member into a tubular structure, and it is very difficult to shape the porous reinforcing support member into a tube having the diameter range of 10-20 mm. For example, when Ni powder having an average pore diameter of 2-3.mu. is molded into a sheet having the porosity of 35% and thickness of 1.0 mm and the sheet is then formed into a tube, such a sheet can not be formed into a tube having a diameter less than 40 mm in diameter by the conventional forming methods.
Even if a multi-layered porous barrier which comprises a sheet-like porous barrier reinforced by a metal porous support tube of large diameter can be formed into a tube of smaller diameter, it is almost impossible to process such a multi-layered barrier having micro-fine pores on the order of several 10 A without inflicting damage to the barrier sheet.
Generally, when a porous tube is employed for any practical application, it is seldom that a single porous tube is employed and a plurality of porous tubes are usually employed in a group as seen in a multi-tube heat exchanger. In such a case, in order to increase the overall surface area of the porous tubes accommodated in a given space, it is necessary to employ porous tubes of small diameter. For example, in the enrichment of uranium, a plurality of units each comprising over ten thousand tubular barriers of 10-20 mm in diameter are employed in a diffusion cell (or unit).
With the above in mind, in the production of a tubular multi-layered porous barrier especially suitable for separation and enrichment of isotopes such as uranium by the gas diffusion system, it is contemplated that a porous support tube of small diameter be prepared by the conventional powder metallurgy method and a powder or powder adhering layer be compressed against the inner and/or outer surfaces of the tube in the thickness of several 10 - several hundred .mu..
In order to form or deposit a porous fine powder layer having a uniform thickness on the order to several hundred - several thousand .mu., such a layer can be formed by either the so-called dry method in which the powder is employed or the wet-method in which slurry is employed. By the wet method, the powder is employed in a slurry form provided by adding distilled water or an organic solvent such as alcohol or acetone and a bonding agent to the powder to render the powder into a paste state. Thus, the slurry has a high fluidity and can be easily formed into a deposition layer of uniform thickness. However, the wet method requires an evaporation step to dry the layer and the wet method has the disadvantage that a crack or cracks will develop in the deposited layer while the layer is being dried. Such crack development is generally known as "the mud crack". In the wet system, the prevention or control of the mud crack presents a very difficult problem.
On the other hand, the dry method has the disadvantage that the powder has a low fluidity and cannot be formed into a layer of uniform thickness. When the fine powder is loosely charged into a tubular porous support member or support tube and the support tube is rotated at a high speed while maintaining the porous tube horizontal, a layer of the powder may be formed having a uniform thickness both in the longitudinal and circumferential directions by a centrifugal force, but the adhesive power of the layer obtained by only a centrifugal force is insufficient and when the rotation of the porous tube is stopped, the uniformity in thickness of the layer would be easily lost.
According to the inventors, it has been found that a layer of fine powder can be firmly deposited on the inner surface of a porous support tube by inserting a flexible rubber tube into the hollow interior of the support tube in a peripherally spaced relationship, charging the fine powder into the annular space between the two tubes, rotating the tube assembly at a high speed while maintaining the tube assembly in a horizontal position to form a uniform powder layer on the inner surface of the support tube, supplying gas under high pressure into the hollow interior of the flexible rubber tube which rotates at the same speed as the support tube during the rotation of the tube assembly to expand the flexible rubber tube radially and outwardly so as to compress the powder against the inner surface of the support tube in a uniform layer.
When a tubular multi-layered porous barrier is to be mounted on a diffusion cell, in order to air-tightly connect the opposite ends of the tubular multi-layered porous barrier to the diffusion cell it is necessary to connect end members to the opposite ends of the multi-layered porous tube.
When the porous barrier layer forming powder is compressed against the support tube and the end members after the end members have been connected to the opposite ends of the barrier tube, and since the porous support tube has pores and a deformation property different from that of the end members, the porous barrier layer will easily develop a crack or cracks therein in the interface between the support tube and the end members. When a crack or cracks develop in the porous barrier layer, separation gas passes through the cracks instead of the barrier layer and then passes transversely through the support tube resulting in the lowering of the separation efficiency of the gas.
When the end members and the porous support tube on which the porous barrier layer is formed are nested within each other, and the members and tube are compressed together with the barrier layer interposed therebetween, the barrier will become very thin and have a low strength, and thus, the barrier will easily crack or separate itself from the support tube in the interface between the support tube and the end members. Similarly, when the porous support tube and the end members are compressed together to compress the powder against the porous support tube so as to form a porous barrier layer on the support tube fixed thereto, the powder present between the support tube and the end members is subjected to a compression force less than that to which the powder is subjected when the powder is compressed against only the porous support tube, because the end members absorb a portion of the compression force by its elasticity, and the thus formed porous barrier layer will have a larger pore diameter than the portion of the porous barrier layer formed on only the porous support tube and will have a lower gas separation efficiency.
Therefore, one principal object of the present invention is to provide a method for producing a novel multi-layered tubular porous barrier which can effectively eliminate the disadvantages inherent in the prior art methods.
Another object of the present invention is to provide a method for producing a tubular multi-layered porous barrier which comprises the steps of depositing fine powder having the average particle size of several 10 - several hundred A onto at least one of the inner and outer surfaces of a sintered tubular porous support member in a layer of uniform thickness to form a fine powder layer, pressuring the powder layer to form a barrier layer, and at the same time, compressing the thus formed barrier layer against the tube.
A further object of the present invention is to provide a method for producing a tubular multi-layered porous barrier in which the formation of said powder layer is carried out by the so-called dry method.
A further object of the present invention is to provide a method for producing a tubular multi-layered porous barrier in which the formation of said powder layer and the compression of said powder layer against said tubular porous support member are carried out by a static pressure application.
A further object of the present invention is to provide a method for producing a tubular multi-layered porous barrier in which the formation of said powder layer is carried out by the so-called wet method.
A further object of the present invention is to provide a method for producing a tubular multi-layered porous barrier in which the end members are compressed against a porous support tube simultaneously with said barrier layer being compressed against the porous support tube.
According to one aspect of the present invention, there has been provided a method for producing a tubular multi-layered barrier which comprises the steps of depositing a fine powder layer onto at least one of the inner and outer surfaces of a sintered tubular porous support member in a layer of uniform thickness, forming a porous barrier layer by compressing the deposited fine powder layer, and at the same time, compressing the barrier layer against said support tube by the application of pressure to the layer.
According to another aspect of the present invention, there has been provided a method for producing a tubular multi-layered porous barrier which comprises the steps of depositing fine powder onto at least one of the inner and outer surfaces of a sintered tubular porous support member in a layer of uniform thickness, forming a porous barrier layer by compressing deposited fine powder layer, and at the same time, compressing said barrier layer against the support tube by the application of pressure to the layer. The application of pressure to the barrier layer also secures the end members fitted in or on the porous support tube to the tube.
The above and other objects and attendant advantages of the present invention will be more apparent to those skilled in the art from a reading of the following detailed description in conjunction with the accompanying drawings which show preferred embodiments of the invention for illustration purpose only, but not for limiting the scope of the same in any way.